The larynx is a vocal organ for communication through a language and vocal folds in the larynx vibrate approximately 100 to 350 times per second with the flow of air of breathing exhaled during vocalization. However, in a disease state in which a voice is changed, a motion of a vocal-fold mucosa is irregular and asymmetric or the vibration decreases and in a severe disease state, the vibration may disappear.
The pressure of air inhaled into a lower airway during the vocalization increases by the vocal folds which are closed at a lower part of a glottis and when the pressure becomes larger than resistance of the vocal folds, the vocal folds are opened while the flow of air that goes from an inferior margin to a superior margin of the vocal folds is formed. A mucosa wave is generated in the vocal-fold mucosa by the flow of the air and a characteristic of a speed or the strength of the mucosa wave determines the quality of a voice. That is, the vocal folds serve to convert sub-glottal pressure generated at a lower portion of the glottis into sound energy while symmetrically vibrating at tens to hundreds of Hz. However, when vocal fold nodules or vocal fold paralysis is caused, symmetricity of the mucosa wave deteriorates and effective conversion of energy becomes impossible to cause trachyphonia (harsh sounds).
Therefore, when abnormality of the voice is examined, determining the vibration of the vocal-fold mucosa, that is, the vibration of the vocal folds is required. To this end, one of methods which are currently used is a laryngeal stroboscope. A method that uses laryngeal videostroboscopy to observe a rapid motion of 100 to 350 times per second of the vocal folds as a slow motion by using the laryngeal stroboscope is primarily used. However, there is a fundamental problem that an image which can be observed through the laryngeal videostroboscopy is an image that does not show vibration of the vocal folds having an actually accurate cycle (period) but is generated by combining some of various cycles. Moreover, meaningful analysis is impossible in a patient having aperiodic dysphonia in which variations among respective periods of the motion of the vocal folds are large or periodic repetitiveness is absent while a gap between the vocal folds is large during the vocalization, and as a result, an overall motion of the vocal-fold mucosa is only qualitatively described. Further, there is a problem to be solved, in that a detailed motion of a part or the entirety or a specific part of the vocal-fold mucosa cannot be individually identified, or the like. In addition, since the image of the laryngeal stroboscopy is only quietly subjectively observed, determination of a skilled experienced person is required for accurate analysis.
As another vocal fold vibration examining method for overcoming a disadvantage, a line scanning videokymography technique has been developed by Svec J G, Schutte H K in 1996. This method is a method that acquires an image for a motion on a consecutive line of approximately 8000 frames per second with respect to a part of the vocal fold arbitrarily selected by an examiner during the examination, that is, one line and displays the acquired image on a monitor. That is, as illustrated in FIG. 1, a part of the vocal folds having a slit is rapidly photographed to photograph a motion of only the part. However, the disadvantage thereof is not observation of the entire vocal folds but evaluation of a part of one line. That is, when an examinee makes vocalization once, only kymogram for one line can be acquired and since a motion of an entire area cannot be observed while acquiring Kymogram, there are problems in that there is no criterion to normally judge distortion by a motion of a patient, and like.
Besides, a method using multi-line videokymography by readjusting an image photographed by an ultra-high speed digital imaging method is also present, but is the same as line videokymography in that the entirety of the vocal folds is not observed and subsidiary equipment such as a high-priced CCD camera which is particularly devised is required, and as a result, there are a lot of limitations in use.